Is the UNLINK command always better than DEL command?

by

Before discussing which one is better, let’s take a look at the difference between these commands. Both DEL and UNLINK free the key part in blocking mode. And the difference is the way they free the value part.

DEL always frees the value part in blocking mode. However, if the value is too large, e.g. too many allocations for a large LIST or HASH, it blocks Redis for a long time. In order to solve the problem, Redis implements the UNLINK command, i.e. an ‘non-blocking’ delete.

In fact, UNLINK is NOT always non-blocking/async. If the value is small, e.g. the size of LIST or HASH is less than 64, the value will be freed immediately. In this way, UNLINK is almost the same as DEL, except that it costs a few more function calls than DEL. However, if the value is large, Redis puts the value into a list, and the value will be freed by another thread i.e. the non-blocking free. In this way, the main thread has to do some synchronization with the background thread, and that’s also a cost.

In a conclusion, if the value is small, DEL is at least, as good as UNLINK. If value is very large, e.g. LIST with thousands or millions of items, UNLINK is much better than DEL. You can always safely replace DEL with UNLINK. However, if you find the thread synchronization becomes a problem (multi-threading is always a headache), you can rollback to DEL.

UPDATE:

Since Redis 6.0, there’s a new configuration: lazyfree-lazy-user-del. You can set it to be yes, and Redis will run the DEL command as if running a UNLINK command.

More Related Contents:

  1. What will be the values of a, b, c?
  2. Linux Blocking vs. non Blocking Serial Read
  3. How to atomically delete keys matching a pattern using Redis
  4. Polling the keyboard (detect a keypress) in python
  5. What’s the Point of Multiple Redis Databases?
  6. MISCONF Redis is configured to save RDB snapshots
  7. How do I delete everything in Redis?
  8. Is non-blocking I/O really faster than multi-threaded blocking I/O? How?
  9. WSL Redis encountered System has not been booted with systemd as init system (PID 1). Can’t operate [closed]
  10. Redis key naming conventions? [closed]
  11. how to get keys which does not match a particular pattern in redis?
  12. How to reset a socket back to blocking mode (after I set it to nonblocking mode)?
  13. Redis command to get all available keys?
  14. Redis: To set timeout for a key value pair in Set
  15. Why is data getting stored with weird keys in Redis when using Jedis with Spring Data?
  16. How can I get all of the sets in redis?
  17. C non-blocking keyboard input
  18. Sending a sequence of commands and wait for response
  19. Scaling Socket.IO to multiple Node.js processes using cluster
  20. How to use redis PUBLISH/SUBSCRIBE with nodejs to notify clients when data values change?
  21. How to reuse redis connection in socket.io?
  22. What does InputStream.available() do in Java?
  23. Is asynchronous jdbc call possible?
  24. how to store a complex object in redis (using redis-py)
  25. Python socket.accept nonblocking?
  26. How to block a mobile number call and message receiving in android application development?
  27. Redis cache vs using memory directly
  28. Non-blocking getch(), ncurses
  29. Maintaining an open Redis connection using BookSleeve
  30. C# Async Serial Port Read

Leave a Comment